Abstract
Sn-W-Cu-Zn-Pb-Ag vein deposits in Japan, belonging to xenothermal class, occur in or in close proximity to regions of rhyolitic (and andesitic) rocks intruded by granitic rocks. The rhyolitic (and andesitic) rocks are products of the same igneous activities as the granitic intrusives. The geologic age of the igneous rocks is late Cretaceous or early Tertiary. The Akenobe, Ikuno, and Tada mines in western Japan and the Ashio mine in eastern Japan belong to the class.The fissures containing vein materials are strike-slip faults and tension cracks formed by the lateral pressure which caused the folded structures in each region. They are also normal faults due to the upheaval and withdrawal (including cauldron subsidence) of the granitic magma.The vein materials at the Akenobe mine are as follows: Ore minerals: cassiterite, wolframite, scheelite, magnetite (lamellar magnetite), arsenopyrite, molybdenite, pyrite, native bismuth, bismuthinite, chalcopyrite, stannite, sphalerite, galena, primary bornite, stannoidite (hexastannite), mawsonite, tennantite, primary chalcocite, roquesite, native silver, argentite, polybasite, stephanite, etc. Gangue minerals are quartz, topaz, apatite, fluorite, orthoclase, chlorite, calcite, rhodochrosite, etc. Indium and selenium are relatively concentrated in the ores from this type of deposit.Similar paragenesis is observed at the Ikuno, Tada, and Ashio mines. At the Ikuno mine, the distribution of the vein materials shows a remarkable zoning. There are zones, vertically from lower to higher and horizontally from center to margin, of (1) tin-tungsten, (2) tin-copper, (3) copper-zinc, (4) zinc-lead, (5) silver. The distribution of the zones generally coincides with the zoning described by Emmons (1924). As a whole it belongs to monoascendant zoning. At the Akenobe mine similar zoning is recognized. It is inferred that at the Akenobe mine the mineralizing fluid may have ascended along the normal faults and have flowed into the tension cracks. At the Tada mine native silver and stromeyerite are characteristically recognized in the silver zone of the deposit. These silver minerals belong to the latest stage of mineralization.At the Ashio mine a reverse zoning is recognized, i.e., the tin-tungsten zone occurs at the higher level and the copper and zinc zones exist below the tin-tungsten zone.From fluid inclusion studies, the decrepitation temperatures of the minerals in the Akenobe mine are determined to be 355 degrees to 155 degrees C. The fluid inclusions in the quartz from the Ashio mine are two-phased (liquid and vapor). The homogenization filling temperatures by the heating microscope, and NaCl equivalent concentration by the cooling microscope, of the fluid inclusions in the quartz from the Ashio mine are 350 degrees to 200 degrees C and 0 to 9 percent, respectively. In some cases, it is observed under the microscope that in the same quartz crystal from the veins of the Ashio mine the fluid inclusion predominating liquid phase coexists with the one predominating vapor phase. This indicates that the boiling phenomenon of the ore-forming fluid has occurred in the course of mineralization.In areas composed of Permo-Carboniferous sedimentary rocks intruded by the granitic rocks of the late Cretaceous or Early Tertiary period which are adjacent to regions of the above ore deposits, there occur hypothermal tungsten and tin veins, and hypothermal or mesothermal chalcopyrite and pyrrhotite deposits. In the hypothermal tungsten and tin veins, primary bornite, stannoidite, and mawsonite are lacking, which are found in the above xenothermal type. In the xenothermal type, cubanite (mackinawite) and pyrrhotite, often found in the hypothermal and mesothermal types, are not recognized. At the Tada mine the following succession of Cu-Fe-Sn-S mineralization is recognized: Pyrite, Chalcopyrite --> Chalcopyrite, Stannite --> Chalcopyrite, Stannoidite --> Chalcopyrite, Bornite, Mawsonite. From hydrothermal experiments mawsonite is apparently stable under conditions of higher sulfur fugacity or lower temperature than the assemblage of bornite + stannoidite + chalcopyrite. Similarly, stannoidite would be stable under conditions of higher sulfur fugacity or lower temperature than the assemblage of stannite + bornite + chalcopyrite. The mineralization sequence at the Tada mine thus proceeds toward lower sulfur fugacity and lower temperatures, depositing the native silver at the latest stage.
